A cross-type universal joint can be assembled in the connecting section between the end section of a drive shaft of the drive system of an automobile and a different rotating shaft, and is capable of transmitting torque between the drive shaft and rotating shaft that are non-linearly arranged. A cross-type universal joint can be assembled between the steering shaft and intermediate shaft, or between the intermediate shaft and pinion shaft of the steering gear unit of a steering apparatus for an automobile, and can transmit torque between these shafts that are non-linearly arranged. Construction of a cross-type universal joint such as disclosed in JP 8-135674 (A), JP 9-60650 (A), JP 2005-48809 (A), JP 2006-29551 (A), JP 2010-181015 (A) and JP 2010-181016 (A) has been widely known.
FIG. 10 illustrates an example of a conventional cross-type universal joint. This universal joint 1 joins a pair of yokes 2a, 2b and one joint cross 3 by way of four shell-type needle bearings 4 so that relative displacement is possible. The yokes 2a, 2b are formed into a forked shape having a pair of arm sections 5a, 5b, and circular holes 6a, 6b are formed on the tip end sections of each of these arm sections 5a, 5b. The circular holes 6a (6b) that are formed on the tip end sections of the same yoke 2a (2b) are concentric with each other. Moreover, the joint cross 3 is formed by uniformly arranging four shaft sections 8 in a radial manner around the outer circumferential direction of a connection base section 7. By supporting the shaft sections 8 inside the circular holes 6a, 6b by way of shell-type needle bearings 4 so that pivotal displacement is possible, the yokes 2a, 2b of the universal joint 1 are connected by way of the joint cross 3 such that pivotal displacement is possible and so that torque can be freely transmitted.
The shell-type needle bearings 4 comprise one shell-type outer ring 9 and a plurality of needles 10. The shell-type outer ring 9 is formed by plastic working such as deep drawing to bend a hard metal plate, such as a carbon steel plate, and comprises a cylindrical section 11, a bottom plate section 12 and an inward-facing collar section 13. The bottom plate section 12 entirely covers one end in the axial direction of the cylindrical section 11 (outside surface side of the arm sections 5a, 5b when assembled in the circular holes 6a, 6b). Moreover, the inward-facing collar section 13 is bent inward in the radial direction from the other end in the axial direction of the cylindrical section 11 (inside surface side of the arm sections 5a, 5b when assembled in the circular holes 6a, 6b), and is curved in a direction so that the surface that faces the needles 10 is a concave surface.
The shell-type needle bearings 4 are assembled with the respective shell-type outer rings 9 fitted into the respective circular holes 6a, 6b with an interference fit and with the shaft sections 8 of the joint cross 3 inserted on the inside of the needles 10. This assembly work is performed with the shaft sections 8 inserted in the circular holes 6a, 6b by press fitting the shell-type needle bearings 4 into the circular holes 6a, 6b from the opening on the outside surface side of the arm sections 5a, 5b. Moreover, the seal ring 14 is fitted and supported beforehand around the base end section of the shaft sections 8 with an interference fit. In the assembled state, the inner circumferential surface of the cylindrical section 11 of the shell-type outer ring 9 functions as an outer ring raceway, and the outer circumferential surface of the shaft section 8 functions as an inner ring raceway.
The seal ring 14 is formed such that an elastic member 16 is reinforced by a metal core 15. The elastic member 16 comprises a radial seal lip 17 and a thrust seal lip 18. When the joint cross 3 and shell-type outer ring 9 are combined and to form the universal joint 1, the edge on the tip end of the radial seal lip 17 comes in elastic contact around the entire circumference of the portion of the outer circumferential surface of the shell-type outer ring 9 near the opening end. In this state, the seal ring 14 seals the inner space of the shell-type outer ring 9 from the external space, and together with preventing lubricant such as grease and the like that is on the inside of the shell-type outer ring 9 from leaking to the outside, prevents foreign matter from getting inside the shell-type outer ring 9 from the outside. The shape of the seal lip, as disclosed in JP 2010-181015 (A) and JP 2010-181016 (A), can have triple seal ring construction by forking the tip end sections of a radial seal lip.
When using a universal joint 1 that is constructed in this way and assembled in the connecting section between the end section of a drive shaft and another rotating shaft, from the aspect of both making the universal less expensive and maintaining durability, there is room for improvement as will be described below. First, in order to reduce costs, carbon steel plate such as case hardening steel or bearing steel having a low procurement cost is preferably used as the metal plate for the shell-type outer ring 9 that is assembled in the shell-type needle bearing 4. However, in the case of carbon steel plate, when used in severe operating conditions, corrosion of the surface cannot be avoided. For example, in the case of assembling a universal joint 1 in the drive system of an automobile, such as in the propeller shaft, corrosive material such as muddy water that is splashed up as the vehicle travels, or water that is mixed with a snow melting agent adheres to the surface of the shell-type outer ring 9, and rusting occurs on this surface.
The portion of the surface of the shell-type outer ring 9 where rusting occurs becomes rough, or in other words, becomes a rough surface having a large friction coefficient, so there is strong friction against the tip end edges of the seal lips 17, 18 of the seal ring 14, and thus wear of these seal lips 17, 18 advances. For example, as illustrated in FIG. 11A, in the initial stage, even when the tip end edges of the seal lips 17, 18 come in contact around the entire circumference of the surface of the shell-type outer ring 9 such that there are no spaces, when rust 19 occurs on part of the surface of the shell-type outer ring 9 as illustrated in FIG. 11B, first, the edge of the tip end of the radial seal lip 17 wears due to rubbing against this rust 19. As a result, a space 20 occurs between the tip end edge of this radial seal lip 17 and the surface of the shell-type outer ring 9. Consequently, corrosive material enters through this space 20 beyond the radial seal lip 17, and as illustrated in FIG. 11C, rust 19a occurs on the surface of the shell-type outer ring 9 that reaches the deep section. Due to rubbing against this rust 19a, the tip end edge of the thrust seal lip 18 also wears, and a space 20a also occurs between the tip end edge of this thrust seal lip 18 and the surface of the shell-type outer ring 9. As a result, the seal by the seal lips 17, 18 decreases or is lost, and it becomes impossible to prevent lubricant from leaking out or prevent foreign matter from getting inside.
As disclosed in JP 2010-181015 (A), rust-proof coating of the surface of the universal joint, including the surface of the shell-type needle bearing, may be performed after assembly. However, in this case, not only is there a possibility that the performance of the seal will decrease due to unevenness of the coating film, but there is also a possibility that as relative displacement occurs between the seal ring and the shell-type outer ring, corrosive material will adhere to the portion of the surface of the shell-type outer ring that is not covered by the coating film, and corrosion will advance from that portion. By coating the enter outer surface of the shell-type outer ring before assembly in the yoke, it is possible to solve this problem, however, the dimension of the outer diameter of the shell-type outer ring increases by the amount of the coating film, so there is a problem in that not only is the work of press fitting the shell-type outer ring into the circular holes on the yoke side hindered, but the dimensional precision after pressure fitting also worsens.
By constructing the shell-type outer ring 9 using stainless steel plate, it is possible to suppress the occurrence of rust 19, 19a, and improve the durability of the universal joint 1 in which the shell-type needle bearing 4 is assembled. However, stainless steel plate is not only more expensive than carbon steel plate, the drawing process for drawing a part that has the necessary hardness is troublesome, and the processing itself is not only difficult, but also increases processing costs. Therefore, in consideration of lowering costs, it is preferred that the use of stainless steel plate be avoided.
JP 2002-146837 (A) and JP 2002-294465 (A) disclose construction wherein a chemically treated coating for preventing rust, such as an insoluble metal phosphate treated film or phosphate treated film, is formed on the surface of the component parts, such as the outer ring, of rolling bearing. These chemically treated films hardly increase the outer diameter dimension of the outer ring, and to a certain extent are expected to be able to prevent rust. In addition, by performing this kind of processing, it is possible during the required period of use of the universal joint to prevent the formation of holes through the shell-type outer ring and prevent corrosion to an extent that would cause the strength to remarkably decrease. However, in the case of severe operating conditions, it would be possible that thin rusting occurs on the surface thereof. Wear of the tip end edge of the seal rip progresses due to the thin rusting. Therefore, in the case of severe operation conditions, the formation of this kind of chemically treated film may not be completely sufficient from both the aspect of maintaining good seal performance of the seal ring over a long period of time, and maintaining durability of the universal joint in which a shell-type needle bearing is assembled.